Sponge iron, metallized pellets, briquettes, lump, or reduced metal materials such as DRI are produced by the direct reduction of ores or metal oxides. Large quantities of metallized iron pellets are made in the direct reduction process wherein particulate iron oxide is reduced substantially to metallic iron by direct contact with a reducing gas such as a mixture of hydrogen and carbon monoxide without becoming liquid. Throughout this specification and appended claims, the term "metallized pellets" is intended to include metal-bearing pellets such as sponge iron, pellets, lumps, briquettes, DRI or other compacted forms of reduced metal and the like which contain at least 80 percent of their metal in the metallic state with the balance substantially in the form of metallic oxides. Metallized in this sense does not mean coated with metal, but means nearly completely reduced to the metallic state. For ease of discussion and visualization, the majority of this specification will describe the invention as it relates to DRI, although it should be understood that the invention will work equally well with other forms of metallized pellets.
A problem associated with the use of hot DRI to make steel or other products is its inherent tendency to oxidize upon exposure to air or water. Throughout this specification, hot DRI refers to material at a temperature in excess of 650.degree. C., and cold DRI refers to material at a temperature less than 100.degree. C. Exposure of a mass of hot active DRI to atmospheric air and moisture causes oxidation of the metal ("rusting") with a significant loss of metallization. The oxidation also produces heat that can dramatically raise the temperature of a mass of DRI. The process of rusting also releases water bound hydrogen into the immediate environment. Under certain conditions, the hot DRI can ignite the liberated hydrogen resulting in additional heat, release of additional hydrogen, and possibly an explosion.
DRI must be removed from the furnace in which it is produced in order to be further processed into steel or other end products. Therefore methods are needed to transport DRI while reducing the risk of oxidation. One method is to cool the DRI to a sufficiently low temperature to prevent the ignition of hydrogen. One drawback to this method is that DRI production systems are typically designed to be "all or nothing" propositions with respect to cooling. Either all of the DRI out of a particular furnace is cooled or none of it is cooled. Given the need for both hot DRI, for example as an energy efficient feed to a steelmaking process, and cool DRI, a need exists for an apparatus and method for taking the output from a shaft furnace and providing both hot and cold DRI for subsequent use.
The problem addressed by this invention has been addressed in U.S. Pat. No. 5,296,015 in which hot DRI is separated into cool DRI and hot DRI. Gas is used to cool DRI descending from a reduction furnace. Initially, the cooling gas is air, however, the gas is recirculated in a hermetic closed system and the oxygen content is quickly removed by reaction with the hot DRI. Thereafter, the recirculated gas is predominately nitrogen with low partial pressure of oxygen and water vapor. The DRI cooler uses a gas recirculation system in which cooling gas is introduced into the lower portion of the container and flows countercurrently to the flow of the DRI. When the DRI is desired, the gas recirculation system is operated and the material is transferred from the lower portion of a bin with a conical lower part by pneumatically transporting the material. The pneumatic transportation is carried out with carrier gas velocities in the range of 9 to 35 m/s at a pressure of 5 kg/cm.sup.2 and a mass to carrier ratio of between 7 and 25. The cooled DRI is removed to storage means for later use as feed stock for various purposes. When hot DRI is wanted, for example to directly fed an electric melter, the cooling gas does not circulate through the descending DRI material. Again the material is conveyed pneumatically to an end use, for example, an electric melter. Unfortunately, the patented invention fails to anticipate the need for a method and apparatus to simultaneously provide sources of hot and cooled DRI, establishing hot DRI routing by operation of various gates and valves and then sending the furnace output pneumatically to the place for end use of hot DRI, for example, a melter. Additionally, this method uses relatively high gas velocities (22 m/s typical) with concomitant requirements for compressors and scrubbers with increased energy use for the same throughput of material. Moreover, using a gas as a carrier implies solid material circulating through the distribution system at velocities approaching the carrier gas velocities. Although the invention asserts that using a tee-connector at elbow turns ameliorates abrasion there, the patented method implies erosion of the containment structures used to move the material.